DE 23 25 609 C2 pertains to a thread-cutting device for sewing machines with a hook rotating in a horizontal plane. A catch thread device, which has an arm movable between the needle plate and the bobbin case of the hook, is fastened on the side of the hook shaft on a shaft parallel to that shaft. A separating finger, a catch hook for the hook thread and a catch shoulder for the needle thread are provided at the free end of the arm, and these catching elements are located essentially in a vertical plane. A cutting edge is provided in the end area of the catch shoulder. The thread-cutting device also has a knife cooperating with the cutting edge, which is fastened at a spaced location from the stitch hole of the needle plate. A hook thread clamp is fastened on the side of the knife. The catching elements of the catch thread device move during the thread separation and thread catching in the space between the jacket surface of the bobbin case and the radially offset and upwardly extending bobbin case stop finger, which engages between two stop cams contained on the underside of the needle plate.
Thread-cutting devices of this type have now been successfully used for more than 20 years. They bring about a reliable cutting of the sewing thread and generate sufficiently long thread ends connected to the corresponding thread reserve, so that a reliable connection of the thread is guaranteed at the beginning of the sewing process. The thread ends remaining on the fabric usually have a length of at least 15-17 mm. Thread ends of this length are in many cases unproblematic. However, there are products in which such long thread ends are disturbing. For example, seam spaces of 10 mm are common in the manufacture of pads and the manufacture of vehicle seats. In these cases, thread ends having a length of 15-17 mm project over the finished product by 5-7 mm, which makes necessary a subsequent finishing, i.e., the cutting off of the projecting thread ends.
U.S. Pat. No. 3,658,021 (corresponding to DE PS 19 41 681) describes a thread-cutting device for sewing machines with a hook rotating in a horizontal plane, in which the needle thread and the hook thread are cut off at a relatively short distance under the fabric. One catch thread device and thread deflector each, designed as circle ring segments, as well as a knife, which is likewise designed as a circle ring segment and is arranged between them, are arranged coaxially to one another for this purpose and are movably mounted in guide grooves of a support plate, which grooves extend eccentrically to the hook. The catch thread device is moved by means of a connecting rod mechanism from the starting position into the catching position, while it penetrates into the needle thread loop and catches the hook thread with a barb. After reaching the reversal position, the catch thread device is returned into its starting position. The fabric-side parts of the needle thread and hook thread are now pressed by the thread deflector moving together with the knife in a direction opposite that of the catch thread device into a groove of the catch thread device and the hook thread is now introduced into a stationary thread clamp. The knife and a cutting edge provided at the end of the groove of the catch thread device meet in the area of the stitch hole and cut the needle thread and the hook thread, and especially short thread ends are obtained on the fabric side. The satisfactory grasping of the sewing threads is guaranteed here by the thread guide of the hook thread having been modified at the bobbin case of the hook to the extent that it extends upward much more steeply in the vicinity of the top finger than in the case of normal, horizontally rotating hooks.
Even though it would be possible to obtain especially short thread ends on the fabric side with this thread-cutting device, which would not require any subsequent finishing operations on the seam and therefore would meet the corresponding requirements of the industry, this thread-cutting device has not been able to become successful in practice. This was certainly due, among other things, to the fact that the hook would have to be modified, so that it would not be possible to use a standard hook in connection with this thread-cutting device. The main reason for the failure is, however, the fact that a thread deflector moving in a direction opposite the direction of the catch thread device is also needed for grasping the threads, in addition to the catch thread device, and that a device comprising circle ring segments movable in relation to one another is highly susceptible to contamination with finishing agent-containing dust generated during sewing, which acts in connection with lubricants, such as abrasive paste and leads to premature wear and possibly to stiff run of the components.